Interest in new protein isolation techniques has been stimulated in recent years by recent advances in genetic engineering and by increased demand from the medical sector for high purity proteins and enzymes. Most needed are recovery processes which are gentle enough for recombinant proteins and constitutive intracellular proteins, and yet are easily adapted to large-scale production. A technique which fulfills both requirements is extraction with aqueous polymer two-phase systems containing either polyethylene glycol and dextran, or polyethylene glycol and salt. The aim of the proposed research is the development of a general, comprehensive theory, based in statistical mechanics of protein solubility in two-phase aqueous polymer solutions with or without salt. The theory is expected to provide insight into the molecular mechanisms responsible for protein partitioning. The objective is to predict free energies, aqueous phase separations and protein partition coefficients as a function of protein and polymer concentrations, protein and polymer molecular weights, protein charge, pH, protein type, polymer type, salt type, ionic strength, temperature, and the binding energy of affinity ligands. The theory should also allow the prediction of salt union and cation partition coefficients. The single protein partitioning results will be test against experimental data on the partitioning of lysozyme, fumarase, malate-dehydrogenase, human serum albumin, and catalase. The multiple protein partitioning results will be tested against experimental data on polyethylene glycol/dextran aqueous solutions containing CO-hemoglobin and human serum albumin. The proposed research is intended to provide a useful framework for the organization of existing data aqueous two-phase extraction and for the optimal design of new biorecovery processes based on this method.